1. Field of the Invention
The present invention relates to a disk storage apparatus equipped with a composite head that includes a read head and write head. More particularly, the invention relates to a method and apparatus for measuring the head gap length of a composite head, suitable for realizing a disk format in consideration of the head gap length as the interval between the read and write heads.
2. Description of the Related Art
In general, in disk storage apparatuses, such as hard disk drives (HDDs), there is a demand for enhancement of disk formatting efficiency. In the prior art, however, a signal delay (hereinafter referred to as a “read path delay”) in a read circuit (read channel) is considered a bottleneck in the effort to enhance the disk formatting efficiency. This is because the distance (gap) between adjacent data sectors must be set sufficiently wide in order to absorb the read path delay and realize data reproduction of continuous data sectors. In light of this, Jpn. Pat. Appln. KOKAI Publication No. 8-315513, for example, discloses a technique (hereinafter referred to as “first prior art”) for dealing with read path delay without widening the gap. The first prior art is characterized in that a reactivation read gate RRG is turned on before a reference read gate RG. In this prior art, a Viterbi decoder and read phase lock loop (PLL) circuit can be reactivated at an earlier stage to perform a pipeline operation.
On the other hand, recent high-recording-density HDDs generally employ a composite head formed of a slider with a read head and write head mounted separately thereon. HDDs with a composite head need to consider the time delay between a read operation by the read head and a write operation by the write head. Jpn. Pat. Appln. KOKAI Publication No. 2002-269703, for example, discloses a technique (herein referred to as “second prior art”) for adjusting, using a delay circuit, the output timing of a write gate (write gate signal) WG and read gate (read gate signal) RG. The output timing is adjusted based on the distance (head gap length) between the read head and write head in the circumferential direction of the disk, and the disk data format. During writing, a period T1 ranging from the time corresponding to the end of each servo region on the disk, to the time of generating a write gate WG is set equal to the time delay corresponding to the head gap length. The period T1 is the sum of periods T2 and T3. The period T2 is the difference between the time corresponding to the end of each servo region and the time of generating a data sector pulse DSP. The period T3 is the difference between the time of generating the pulse DSP and the time of generating a write gate WG. On the other hand, during reading, a read gate RG is generated at the time corresponding to the middle point of the spin variation gap (pad data) defined from the end of each servo region. As a result, in the second prior art, the information recorded in the end portion of each servo region (i.e., the end portion of servo data) is prevented from being overwritten by a write operation.
In the first prior art HDD, the problem of read path delay is solved, therefore the disk formatting efficiency depends upon the write data length. On the other hand, in the second prior art, while the information recorded in the end portion of each servo region is prevented from being overwritten, it is not necessary to widen the spin variation gap of the servo region end portion. However, in the second prior art, no consideration is given to the problem concerning write data that is recorded in the end portion of a servo region and has a data length corresponding to the head gap length.
As is apparent, a data format made in light of the common difference in head gaps (i.e., the allowable maximum head gap length) is required to prevent the end portion of servo data from being overwritten by a write operation. However, in accordance with recent increases in the recording density of HDDs, the data length corresponding to the same head gap length has increased compared to the conventional data length. Therefore, the influence of the head gap of a composite head upon the efficiency of formatting (i.e., format loss) has become unignorable.